Conventionally, a configuration in FIG. 2 is widely known as such type of radio frequency power amplifier (see patent document 1). There is used a plurality of GaAs Hetero-junction Bipolar Transistors (Q1 through QN) as high-frequency amplifying transistors. The hetero-junction bipolar transistor is hereinafter referred to as HBT or simply as a transistor.
As shown in FIG. 2, an emitter of each transistor (Q1 through QN) is grounded. A base connects with a high-frequency input terminal via a capacitor (C1 through CN). A collector connects with a high-frequency output terminal. The base of each transistor is connected to a base bias circuit via a resistor (R1 through RN). The base bias circuit comprises an HBT (QB1) and a resistor RB1. An emitter of the transistor QB1 is grounded. A base and a collector are connected to each other. These are connected to constant voltage supply VCC via the resistor RB1.
According to the prior art (patent document 1), a high-frequency signal supplied to an input terminal RFin enters each base of the HBT (Q1 through QN) via capacitors C1 through CN, is amplified in each HBT, and exits from a collector of each HBT to an output terminal RFout. The bias circuit supplies a specified voltage. The resistors R1 through RN function as ballast resistors. When a large proportion of current starts flowing through a specific HBT, a large amount of current flows to a ballast resistor connected to that HBT to cause a voltage drop. The base vogltage for that HBT drops to solve the unbalanced current flow.
[Patent document 1]
U.S. Pat. No. 5,629,648